The theory of endosymbiosis suggests that the ancestors of mitochondria were a) formed due an error in cytokinesis. b) once free-living heterotrophic prokaryotic cells. c) parasitic cells that lived within more complex organisms. d) once free-living cells that remained joined after mitosis.

The theory of endosymbiosis suggests that the ancestors of mitochondria were once free-living heterotrophic prokaryotic cells. They were able to use oxygen to release energy from organic molecules by cellular respiration. At some point, they were engulfed by larger prokaryotes, and remained there, providing ATP to the larger cell.